On his new deal starting in 2019-20, Rosen can be sent down to the Marlies without any cap ramifications for the Leafs as his one-way salary is below the $1,050,000 threshold, but as a 25-year-old (as of February) who will be two years removed from his first NHL contract, he would be waiver eligible on his new deal starting next Fall.
With the one-way deal, Rosen will get a considerable pay raise to stay with the organization after making less than he could’ve earned in Sweden on a two-way entry-level base salary worth $70,000 (plus a $92,000 signing bonus) during 2017-18 and for the remainder of 2018-19.